Ordered nanoscale domains by infiltration of block copolymers

ABSTRACT

A method of preparing tunable inorganic patterned nanofeatures by infiltration of a block copolymer scaffold having a plurality of self-assembled periodic polymer microdomains. The method may be used sequential infiltration synthesis (SIS), related to atomic layer deposition (ALD). The method includes selecting a metal precursor that is configured to selectively react with the copolymer unit defining the microdomain but is substantially non-reactive with another polymer unit of the copolymer. A tunable inorganic features is selectively formed on the microdomain to form a hybrid organic/inorganic composite material of the metal precursor and a co-reactant. The organic component may be optionally removed to obtain an inorganic feature s with patterned nanostructures defined by the configuration of the microdomain.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 15/332,335, filed Oct. 24, 2016, which is a divisional of U.S.patent application Ser. No. 13/209,190, filed Aug. 12, 2011, now U.S.Pat. No. 9,487,600, which claims priority to U.S. Provisional PatentApplication No. 61/374,349, filed Aug. 17, 2010, the contents of all ofwhich are incorporated herein by reference in their entireties.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.DE-AC02-06CH11357 awarded by the United States Department of Energy toUChicago Argonne, LLC, operator of Argonne National Laboratory. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to a method of preparing ordered nanoscaledomains. More specifically, this invention relates to a method ofpreparing inorganic ordered nanoscale domains by the infiltration ofblock copolymers with a plurality of precursors.

BACKGROUND OF THE INVENTION

This section is intended to provide a background or context to theinvention that is, inter alia, recited in the claims. The descriptionherein may include concepts that could be pursued, but are notnecessarily ones that have been previously conceived or pursued.Therefore, unless otherwise indicated herein, what is described in thissection is not prior art to the description and claims in thisapplication and is not admitted to be prior art by inclusion in thissection.

Patterned nanoscale inorganic materials with controllable characteristicfeature size, symmetry, and properties are of considerable interest in awide range of fields. However, as feature dimensions shrink below 50 nm,conventional top-down lithographic patterning methods typically sufferfrom slow processing speeds and high costs. To date, wide-scaleimplementation of applications for nanomaterials has been hindered bylimitations associated with production. Molecular-level control in thesynthesis of nanomaterials with precisely tunable properties is highlydesired for mass production of nanoscale devices. Equally important inproduction is low-cost fabrication of periodic nanoscale features overlarge areas. Using conventional methods, these twin goals are generallyat odds with each other.

An alternative, and less expensive approach, is to employ a processanalogous to the biomineralization process and use self-assembledorganic structures as growth-directing agents to guide the synthesis ofinorganic materials into the desired morphology. For example, blockcopolymers (BCPs), which have two or more chemically dissimilarhomopolymers joined together through covalent bonds, can self-assembleinto ordered periodic nanostructure configurations (e.g. spheres,cylinders, lamellae and bicontinuous structures) under appropriateconditions due to microphase separation. Useful devices can befabricated from ordered block copolymer structures by tuning thematerial properties of the two polymer domains. Although the propertiesof the component polymers can be adjusted prior to forming the ordereddomains using organic synthesis, this may affect the phase separation ofthe polymers and prevent formation of the desired nanostructure.

BCPs have offered a relatively easy, inexpensive, and versatile platformfor templating inorganic materials growth. A variety of inorganicmaterials have been self-assembled on BCPs for localized selectivegrowth of such materials in the desired domains, which can act asnanoreactors to physically confine the growth, generally throughhydrophobic forces. However, using conventional techniques, thedimensions of the templated materials are determined by the physicalsize of the original domains in the BCP scaffold, limiting theflexibility of these methods. Moreover, the loss of selectivity fromuncontrolled homogeneous reactions cannot be fully prescribed,especially for reactions involving hydrolytic unstable precursors suchas titania and other technologically important metal oxides. Moreimportantly, the localized material growth in the targeted domains isnot controllable on the molecular level, which is vital for assuringlarge-scale uniformity in mass production of organized nanoscalematerials with precisely controlled material properties.

SUMMARY

The present invention provides processes for preparing nanostructuresand offers a high degree of molecular-level control while maintaininglarge-scale uniformity and tunable modularity of the nanostructures.Molecular-level management of reactions is achieved by a self-limitedinteraction of metal precursors with a self-assembled block copolymer(BCP) scaffold. Using molecular recognition and organized assemblycharacteristics and BCPs, several of the difficulties associated withvarious conventional nanofabrication processes can be overcome. In someembodiments, sequential infiltration synthesis (SIS), a method relatedto atomic layer deposition (ALD) is used for preparing inorganicfeatures with patterned nanostructures on the BCP scaffolds.

The present approach utilizes the polymer chains in well-defined BCPdomains as the molecular scaffold for templating inorganic materialsgrowth through a highly controllable molecular assembly process. Throughthe design of the BCP scaffold and selection of the synthesisparameters, patterned designer materials with controlled size, spacing,symmetry, and composition can be synthesized. Moreover, the processescan yield desirable nanoscale structures at low cost. Potentialapplications for these methods and system extend to virtually alltechnologies in which periodic nanomaterial structures are desirable,including photovoltaics, sensors, membranes, photonic crystals,dielectric materials, and electronics.

In one BCP system, polystyrene-block-poly (methyl methacrylate)(PS-b-PMMA) is presented as an illustrative example. However, theinvention is not so limited and the present methodology is readilyextended via the virtually limitless variety of chemistries availableboth in BCPs and in ALD or SIS.

In an embodiment, a method of preparing a plurality of tunable inorganicpatterned nanometer-scale features by infiltration of a block copolymerscaffold comprises providing a block copolymer scaffold of at least afirst polymer and a second polymer and includes a plurality ofself-assembled periodic polymer nanostructures. The method furthercomprises selecting a first metal precursor that is configured to reactwith the first polymer but is substantially non-reactive with the secondpolymer. A second co-reactant precursor configured to react with thefirst precursor is also selected. At least one cycle on the blockcopolymer scaffold is performed. A cycle comprises exposing the blockcopolymer scaffold to the first metal precursor to react the first metalprecursor with the first polymer and exposing the block copolymerscaffold to the second co-reactant precursor to react with the firstmetal precursor to form an inorganic material on (within) the firstpolymer. This embodiment can be executed in either two-dimensional(single-layer) or three-dimensional (multi-layer) structures. In somecases an initial cycle can be used to deposit an initial layer thatsubsequently serves as a seed for cycles of a different chemistry.

In another embodiment, a method of preparing an inorganic orderednanoscale domain through a self-limited reaction within a blockcopolymer comprises providing a block copolymer with a plurality ofordered polymer nanoscale domains, which are characterized by a reactivefunctional group. The method further comprises selectively binding afirst precursor to the reactive functional group in a self-limitedreaction. Next, a second precursor is reacted with the bound firstprecursor to form an inorganic feature that is localized on theplurality of nanoscale domains within the block copolymer. The blockcopolymer may then optionally be removed in order to obtain a pluralityof the inorganic features, substantially free of the copolymer, thathave a structure defined by the configuration of the plurality ofnanoscale domains within the block copolymer prior to its removal. Inthe described embodiment, the block copolymer is substantially free ofthe reactive functional group outside of the ordered polymer nanoscaledomains. This embodiment can be executed in either two-dimensional(single-layer) or three-dimensional (multi-layer) structures. In somecases an initial cycle can be used to deposit an initial layer thatsubsequently serves as a seed for cycles of a different chemistry.

In yet another embodiment, a nanocomposite organic/inorganic materialcomprises a block copolymer that includes a first polymer and a secondpolymer covalently bonded to the first polymer. The first polymerdefines a plurality of self-assembled ordered microdomains disposedwithin the second polymer. The first polymer includes at least onefunctional group absent from the second polymer. The nanocompositematerial further includes an inorganic material that is substantiallyembedded with each of the plurality of ordered microdomains. Theinorganic features comprise at least one metal selectively bound to atleast one functional group. The at least one metal does not bind to thesecond polymer. This embodiment can be executed in eithertwo-dimensional (single-layer) or three-dimensional (multi-layer)structures. In some cases an initial cycle can be used to deposit aninitial layer that subsequently serves as a seed for cycles of adifferent chemistry.

These and other advantages and features of the invention, together withthe organization and manner of operation thereof, will become apparentfrom the following detailed description when taken in conjunction withthe accompanying drawings, wherein like elements have like numeralsthroughout the several drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c represent a schematic depiction for templated synthesis ofnanoscopic inorganic materials on a single-layer PS-b-PMMA substrate bySIS;

FIGS. 2a-2f show field emission scanning electron microscope (FESEM)images of Al₂O₃ patterns formed on a PS-b-PMMA scaffold disposed on asilicon (Si) wafer formed by various iterations of SIS cycles of Al₂O₃with an exposure/purge timing sequence (60/300/60/300 seconds) followedby an O₂ plasma treatment to remove the PS-b-PMMA scaffold, with FIG. 2ashowing 1 SIS cycle, FIG. 2b showing 2 SIS cycles, FIG. 2c showing 3 SIScycles, FIGS. 2d and 2f showing 6 SIS cycles, and FIG. 2e showing 10 SIScycles; FIG. 2f depicts a mixture of Al₂O₃ nano-posts and nanocylindersobtained from polymer domains with different orientations; and FIG. 2his a plot of the concentration of Al obtained from energy dispersiveX-ray spectroscopy (EDX) taken along the path depicted in FIG. 2 g;

FIG. 3 is a plot of cylinder diameter (depicted by black squares ▪) andcenter-to-center cylinder spacing (depicted by black diamonds ♦) withincreasing Al₂O₃SIS cycles and the corresponding Al₂O₃ featuresthickness (depicted by white squares □) on a separate Si wafer;

FIGS. 4a and 4b are FESEM images of a pattern of TiO₂ cylinders on a Siwafer, formed by 5 and 10 cycles, respectively, of TiO₂ SIS; FIG. 4c isan atomic force microscopy (AFM) height image of the pattern resultingfrom 5 cycles of ZnO SIS on a PS-b-PMMA scaffold; FIG. 4d is an AFMheight image resulting from 1 cycle of Al₂O₃ ALD followed by 3 cycles ofZnO SIS;

FIG. 5a-5d are FESEM images of nanopatterns formed on a Si wafertemplated by a PS-b-PMMA scaffold following 1 cycle of Al₂O₃SIS followedby 10 cycles of tungsten (W) SIS (FIG. 5a ); 1 cycle of Al₂O₃SISfollowed by 20 cycles of W ALD (FIGS. 5b-5c ); and 2 cycles of Al₂O₃SISfollowed by 20 cycles of W SIS (FIG. 5d ); and

FIG. 6a-6b schematically depict the bonding interaction of TMA andTiCl₄, respectively, with a carbonyl group disposed within a blockcopolymer scaffold.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention provides molecular-level control for preparinginorganic patterned nanostructures of a broad variety of materials withtunable characteristic feature sizes and shapes by utilizing thecapabilities of block copolymer self-assembly and the selectiveinteraction of one or more metal precursors with particular polymerunits of the block copolymer. The resulting materials have a number ofpotential uses, including photovoltaic devices, structural supports fora separation membrane in a battery, a fluid filtration membrane,filtering and/or guiding selected wavelengths of light, as activechannel material in a transistor, as an array emitter or a fieldemitter, a three-dimensional electrical contact, and a catalyst. Inphotovoltaic devices, nanostructures comprised of light absorbers,charge separation materials, and/or charge transport materials canoutperform analogous devices without nanostructures—this is especiallytrue in systems where bound excitons are formed such as in organic andhybrid organic/inorganic photovoltaics. The uniformity and tenability ofspacing between nanostructures lends itself to separation andnanofiltration applications. Photonic crystals with uniformly spacednanostructures are effective at manipulating and modulating light.

In some embodiments, the method uses sequential infiltration synthesis(SIS), SIS is related to atomic layer deposition (ALD). In general, theSIS process involves exposing the patterned organic material layer tovarious gas phase precursors to synthesize inorganic features. SIS coatsthe surface of the organic material but also infiltrates into the bulkorganic material as well by adjusting the gas phase exposure pressureand timing. The gas phase precursors are available for selection for SISmodification which are capable of forming inorganic components within avariety of organic materials. Examples of inorganic features prepared bySIS include Al₂O₃, TiO₂, ZnO, SiO₂, and W. The infiltration of theinorganic material may be confirmed by monitoring mass intake by quartzcrystal microbalance or by detecting diffusion using cross-sectionalanalysis with EDX or other techniques.

In some embodiments, the SIS method may include relatively long periodsof gas phase exposure to precursors. For example, the SIS method mayinclude a relatively long period of gas phase exposure to precursor Afollowed by a long period of exposure to precursor B (with a purgingstep in-between). In another embodiment, the method may include a seriesof short pulses of A followed by another series of short pulses of B(with a purging step in-between). In some embodiments, a series of shortpulses may be combined with long periods of gas phase exposure to aprecursor. In some embodiments, the total time of exposure to aprecursor for a SIS cycle may be 5 to 25 times higher than the typicaltime for an ALD cycle. In some embodiments, the total time of exposureto a precursor for a SIS cycle may be 10 times higher than the typicaltime for an ALD cycle. In some embodiments, the SIS method may includeuse of high pressure to facilitate infiltration of the inorganicmaterial.

Various embodiments of the present invention utilize sequentialinfiltration synthesis (SIS), which is related to atomic layerdeposition (ALD) to form patterned inorganic features in a blockcopolymer scaffold. In an embodiment, a block copolymer scaffold isexposed alternately to two reactive gases. The first reactive gas may bea metal precursor that is selectively reactive with a functional grouppresent in one of the polymer units but absent from at least one of theother polymer units in the block copolymer. The metal precursorselectively binds (either covalently or non-covalently) to thefunctional group but is substantially non-reactive with one or moreother polymers. Substantially non-reactive refers to no greater than 10%polymer/precursor reaction as compared with the reaction between thetarget polymer and precursor. The metal precursor is substantiallynon-reactive when the precursor does not bind to the polymer under a SISreaction. The second reactive gas may be a co-reactant—for example,serving as a second precursor in a cycle—that is selectively reactivewith the first precursor that is bound to the polymer unit. By way ofexample, the first reactive gas may be a ligated metal such as trimethylaluminum (TMA) and the second reactive gas may be water. In someembodiments, a third precursor may be used.

Block copolymers are molecules composed of two or more polymersconnected with covalent bonds. For example,polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) is composed ofpolystyrene (PS) and poly(methyl methacrylate) (PMMA) covalently linked.By varying the preparation conditions of the block copolymer, theseblocks will phase-separate and self organize into structures withordered nanoscale domains in various configurations such as spheres orlamella. The characteristic shape and dimensions of these domains can betuned via polymeric design. In some embodiments, the block copolymerincludes a plurality of self-assembled periodic polymer nanostructures.The nanostructures such as cylinders assemble on their own into arepeated pattern, such as by phase separation, which can, in some cases,be directed or manipulated with external parameters such as temperature,solvent vapors, electric fields, mechanical forces, magnetic fields,flow, or surface topography.

In general, on a scaffold surface with the correct chemical termination,the SIS process results in the growth of an inorganic featuresassociated with the SIS precursors used. The inorganic features is aninorganic layer that may be between 0.2 nm to 500 nm in height. Forexample, an aluminum oxide (Al₂O₃) features may be formed on thescaffold surface using a TMA precursor and a water co-reactantprecursor. However, where the appropriate chemical termination is absentfrom the scaffold surface the SIS of the precursors will be inhibited.By selecting the precursors to selectively react with only one of theblock copolymer units, the SIS process will result in growth within thatpolymer component only. Thus, SIS using TMA and water precursors on aPS-b-PMMA scaffold results in growth of inorganic Al₂O₃ features almostexclusively within the PMMA component, to the exclusion of the PScomponent. In some embodiments, less than 10% of Al₂O₃ growth may be inthe PS. The ALD precursors infiltrate molecular-scale voids in the blockcopolymer scaffold and attach (covalently or non-covalently) to thepolymer chains to form an inorganic-organic hybrid composite material.For example, TMA will react with the oxygen species in the PMMAcomponent but not with the PS, which is comprised solely of carbon andhydrogen. An inorganic feature, in this case an Al₂O₃ feature,substantially assumes the same ordered nanostructure/spacing as theblock copolymer, although the width of individual domains can be furthertuned using the number of SIS cycles. The composite coated polymerscaffold can be left intact, or the sample can be oxidized to remove theorganic material, leaving only the inorganic SIS material but preservingthe initial structure provided by the block copolymer in the inorganiccomponent.

In light of the broad range of selective chemistries between variousmetal precursors suitable for ALD/SIS and polymer units as well as theenormous library of block copolymers and ALD-related processes,patterned functional materials could be synthesized onto a broad rangeof scaffolds. The process may be generalized to designing the desiredblock copolymers in terms of materials and microdomain configuration(e.g., shape and dimensions) and selecting corresponding SIS precursorsreactively compatible with the appropriate polymer units andcharacterized by the desired final material properties (e.g., electronicand/or photo response). The process may used in forming variousinorganic materials, including a metal, metal oxide, a metal nitride, ametal sulfide or other metal chalcogenide, a metal carbide, or a metalphosphide. In various embodiments, a transparent conductive metal oxidesuch indium tin oxide (ITO) may be formed using ALD precursors known inthe art.

For example, following the present approach, various inorganic featureshave been selectively deposited within block copolymers, including theformation of ZnO, TiO₂, and W features. These materials grow exclusivelyon the PMMA unit of the PS-b-PMMA via selective reaction with thecarbonyl chemistry of the PMMA. However, the precursors associated withthe features are non-reactive with PS, which has no carbonyl groups. Foreach material, the inorganic material assumes the self-assembled,periodic nanostructure of the PMMA within the copolymer scaffold. Thus,an inorganic material is selectively nanopatterned and anorganic/inorganic hybrid composite material is formed.

Although carbonyl functional groups are described as one example of apolymer component or reactive functional groups that may be utilized forselective inorganic material growth, a variety of different polymerunits are available to interact with various metal precursors throughvarious interactions, including metal-ligand coordination, covalentbonding, and other interactions. For example, the pyridine groups inpolyvinylpyridine, a common block for BCPs, could be used to selectivelybind various metal compounds including Al(CH₃)₃, AlCl₃, ZnCl₂, CdCl₂,etc., which may be used as precursors in ALD-related processes.Additionally, hydroxyl groups provided by polyacrylic acid, anothercommon block for BCPs, could react with various metal precursors,including Al(CH₃)₃, TiCl₄, Zn(C₂H₅)₂, etc. to form covalent bonds.

Two components are significant in driving the present processes toobtain particular material characteristics. The first component is theselective and self-limited reaction of a metal precursor such as TiCl₄,SnCl₄, AlCl₃, Al(CH₃)₃, etc., which are Lewis acids in this example,with strategically selected functional moieties in the BCP such as thecarbonyl groups in PMMA microdomains. Once bound to the polymer, thegrafted metal-ligands serve as nucleation sites for the secondcomponent, which is the inorganic material synthesis by SIS. Within eachof these components, the reactions are controllable on the molecularlevel and the characteristic self-limited heterogeneous reactionsprovide macroscopic uniformity in principle.

A broad range of organized nanomaterials with tunable dimensions can besynthesized from a BCP scaffold such as PMMA and an even more expansiverange is available in the context of other BCP chemistries. Todemonstrate this approach with a particular set of materials, organizedAl₂O₃ and TiO₂ nanocylinders with controllable dimensions weresynthesized. Starting with a monolayer of —Al—OH seeds generated in thePMMA domain via Al₂O₃SIS.

FIG. 1 depicts a technique for patterning a plurality of inorganicnanoscale features onto a scaffold by performing SIS with aself-assembled PS-b-PMMA block copolymer features template. The term“self-assembled” refers to both spontaneous organization to the BCP intonanoscale domains such as PMMA cylinder within a PS matrix or directedor biased self assembly such as by the use of an electric field or atemperature/pressure gradient. At 10 a PS-b-PMMA scaffold 100 isprovided on a process scaffold 101, which may be a Si wafer, and loadedinto the ALD reactor. The PS-b-PMMA scaffold 100 comprises a pluralityof orientated PMMA microdomains 102 having one or more definedconfigurations, cylinders in the depicted embodiment. The plurality oforientated PMMA microdomains 102 are disposed within a matrix of PS 103.

At 11 the PS-b-PMMA scaffold 100 is exposed to a vapor of a metalprecursor, which diffuses into the BCP features and selectively reactsspecifically with carbonyl groups in the PMMA domains as depicted inFIG. 6A. The non-coordinated excess metal precursor is then removed fromthe domain by a purge step such as with high purity N₂ to preventnon-self-limited, homogeneous reactions. The PS-b-PMMA scaffold 100 isthen exposed to a co-reactant precursor such as water, which reacts withthe coordinate metal precursor. The first monolayer of a coordinatedmetal precursor provides reactive sites for the subsequent SIS process,which selectively grows an inorganic material 104 within the active PMMAmicrodomains 102. As the SIS process operates in a self-limitedheterogeneous surface reaction mode, the growth of materials in the PMMAmicrodomain 102 continues in bottom-up assembly fashion with molecularprecision. At 12, the polymer template 100 may be removed by thermalannealing or plasma treatment or other process known in the art. Theresult is a patterned inorganic structure 105 that mimics theconfiguration of the original self-assembled PMMA microdomains 102.

Because the assembly process of the present technique uses units on thepolymer chains as the molecular template, the final domain size of thedeposited inorganic material is mainly determined by a combination ofthe number of available reactive sites in the domain and the amount ofmaterial being assembled into the domain by SIS cycling, an SIS cyclecomprising exposure of the scaffold to the metal precursor and thefollowing co-reactant exposure. This combination of attributes offerssignificant flexibility in tuning the final feature size when comparedwith conventional methods. For example, the process is capable offabricating features considerably narrower than the characteristicdimension of the scaffold template. Additionally, the separatedheterogeneous surface reactions in this process greatly decrease thelikelihood of uncontrollable overgrowth in undesired microdomains.

With reference to FIGS. 2a-2e , a series of SEM images show patternedAl₂O₃ nanocylinders resulting from 1 (FIG. 2a ), 2 (FIG. 2b ), 3 (FIG.2c ), 6 (FIGS. 2d and 2f ), and 10 (FIG. 2e ) SIS cycles of Al₂O₃ with aPS-b-PMMA features over a silicon scaffold. The figures show thenanocylinders after the self-organized PS-b-PMMA BCP scaffold thinfeatures were removed by O₂ plasma treatment or by heating in air at500° C. for 6 hours. The O₂ plasma etching was performed at 50 W for 1minute in a March CS-1701 plasma etcher.

In some embodiments, sequential infiltration synthesis (SIS) can beutilized to provide the described stepwise growth. In certainembodiments, finely tuned processing conditions, complex composites andinorganic nanomaterials with tunable features of hierarchical scales canbe synthesized for applications ranging from solar cells to lithiumbatteries to catalysis. In-situ diffusion and reaction studies using aquartz crystal microbalance (QCM) show that even though SIS is aself-limited stepwise growth process, the SIS process depends stronglyon complex coupled diffusion and reaction processes, whichdifferentiates it from traditional atomic layer deposition. For example,in-situ QCM data shows that for polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) block copolymer (BCP) features, the reactantsdiffuse to the reactive site in PMMA through PS domains. The PS domains,being unreactive to the precursors act as a highway for delivering thosereactants and remain unobstructed throughout the SIS process. Becausethe reaction between metal precursor and PMMA influences the mobility ofmolecular chains of PMMA, the diffusion of metal precursors stronglydeviates from the classic diffusion model derived from Fick's Law. Inother words, the presence of the PS matrix allows for an alternativepathway for the inorganic material to infiltrate the BCP even after thediffusion pathways are blocked by repeated cycles.

In one embodiment, by tuning the strength of the reaction between metalprecursors and soft matter (e.g. polymers), the interface between theSIS-synthesized materials and soft matter can be precisely adjusted, aswell as the final structure of the composites and templated inorganicmaterials. For example, poly (4-vinyl pyridine) presents strongcoordination reaction with TiCl₄ and Al(CH₃)₃, which prevents furthermaterial diffusion into the polymer and results in hollow tubes andspheres. For PMMA scaffolds, the reaction between metal precursors andsoft matter is not strong enough to block the diffusion of materialsinto the matrix of PMMA nanodomains, therefore, solid nanorods,nanowires, or nanoposts with precisely-controlled size can be generated.

Examples

Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA,Mw=50,500/20,900) (Polymer Source, Inc.) was purified through Soxhletextraction to remove excess PS homopolymer. BCP solutions were preparedin toluene (Fisher, 99.5%) with a concentration of 13 mg/mL. ThePS₄₈₅-b-PMMA₂₀₁ block copolymer scaffold features were prepared by spincoating from a toluene solution onto cleaned silicon scaffolds withnative SiO₂. After deposition, PS-b-PMMA features were annealed at 250°C. for two hours in a tube furnace under a flowing Ar atmosphere, thencooled to room temperature to obtain self-assembled patterns. Thein-plane PMMA cylinders were 30±3 nm in diameter, and thecenter-to-center lateral distance was 60±5 nm. These dimensions can bevaried by adjusting the molecular weight of the constituent polymerblocks.

The Al₂O₃SIS process was performed using the SIS timing sequence:60/300/60/300 seconds, where the first times represent first metalprecursor trimethyl aluminum (Al(CH₃)₃ TMA 96%) exposure, inert purge,second co-reactant precursor (water) exposure, and inert purge.Ultrahigh purity N₂ (99.999%) was used as the purge gas and carrier gaswith further purification by an inert gas filter (Aeronex Gatekeeper)before entering the reactor. All precursors were introduced into the ALDreactor at room temperature vapor. In order to remove moisture andachieve thermal equilibrium, the scaffolds were subjected to a 300 sccmN₂ flow at 1 Torr for at least 30 minutes and then evacuated to lessthan 20 mTorr before commencing SIS.

As depicted in FIGS. 2a-2e , each of the Al₂O₃ stripe patterns resemblethe parallel-oriented cylindrical microdomains of PMMA that were presentin the original PS-b-PMMA scaffold thin features. FIGS. 2a-2f show fieldemission scanning electron microscope (FESEM) images of Al₂O₃ patternsformed on a PS-b-PMMA scaffold disposed on a silicon (Si) wafer formedby various iterations of SIS cycles of Al₂O₃ with an exposure/purgetiming sequence (60/300/60/300 seconds) followed by an O₂ plasmatreatment to remove the PS-b-PMMA scaffold, with FIG. 2a showing 1 SIScycle, FIG. 2b showing 2 SIS cycles, FIG. 2c showing 3 SIS cycles, FIGS.2d and 2f showing 6 SIS cycles, and FIG. 2e showing 10 SIS cycles; FIG.2f depicts a mixture of Al₂O₃ nano-posts and nanocylinders obtained frompolymer domains with different orientations Additionally, the spatiallylocalized presence of Al was verified by energy dispersive X-rayspectroscopy (EDX) as illustrated in the plot of FIGS. 2g and 2h , whichillustrate a concentration of Al disposed along the plurality ofnanocylinders. Some of the PMMA features are oriented in the plane ofthe field and other are standing on one end and appear to be chains anddots in FIG. 2(f).

Contraction of the polymer and aggregation of the Al₂O₃ nuclei duringthe O₂ plasma etching process further explain the formation of Al₂O₃cylinders. The diameter of the Al₂O₃ cylinders resulting from oneTMA/H₂O cycle is 8.48±1.54 nm, which is much smaller than theapproximately 30 nm-wide PMMA domains in the BCP features but also muchbigger than expected in view of the Al₂O₃ ALD growth rate on a planarsurface (1.2 Å/cycle). According to the self-limiting behavior ofreaction between TMA and carbonyl groups, the maximum number of TMAmolecules coordinating to each PMMA domain is determined by the numberof carbonyl groups, which is about 200 per PMMA chain in this case.Assuming all 200 carbonyl units are coordinated to TMA, the cylinderdiameter resulting from the first TMA/H₂O SIS cycle would be about 11.5nm, which is slightly higher than the experimentally observed value. Theactual smaller size of the Al₂O₃ cylinders is attributable to areduction in the number of coordinated TMA molecules by steric effectsof the ligands in grafted TMA and the polymer scaffold and folding ofpolymer chains. These mechanisms are further supported by the brokenpoints (as indicated by the arrows in FIG. 2a ) along the Al₂O₃cylinders generated from a single TMA/H₂O cycle. Significantly, theslightly smaller Al₂O₃ feature size relative to the theoretical value isindicative of a reaction templated by the carbonyl groups on the polymerchain; and the coordination process of TMA onto the carbonyl groups isindeed self-limited. These characteristics provide the desiredmolecular-level control of the present processes.

With the Al—OH nucleation sites formed in the first cycle through thecoordination reaction between TMA and the carbonyl groups and thefollowing hydrolysis reaction with H₂O, traditional Al₂O₃ ALD chemistrycould be performed to incorporate more Al—O ligands into the domain in aself-limited layer-by-layer fashion. With increasing Al₂O₃ cycles, thediameter of the Al₂O₃ cylinders increases as deduced from FIGS. 2a-2e .After 10 cycles, the Al₂O₃ cylinder diameter increases to 30.8±2.7 nm,which is comparable to the original PMMA domain size. The Al₂O₃cylinders become continuous for samples with more than 1 TMA/H₂O cycleas evidenced by the absence of visible breaks in the cylinders depictedin FIGS. 2b -2 e.

With reference to FIG. 3, the cylinder diameters as well as thecenter-to-center spacings measured from the SEM images (FIGS. 2a-2e )are plotted against the number of cycles. The mean center-to-centervalue remains nearly constant, at about 60 nm, regardless of the numberof cycles performed. This value is consistent with the center-to-centerspacing between PMMA domains in the initial BCP thin features template,60 nm±5 nm.

The linear dependence of the Al₂O₃ cylinder size on the number of cyclesdepicted in FIG. 3 highlights the capability of ALD to tune theinorganic feature size without changing the dimensions of the BCPtemplate. This eliminates the need to prepare BCPs with different domainsizes to control the size of the nanostructure, therefore providingsubstantial flexibility and simplifying the process. The increase ofAl₂O₃ cylinder diameter results directly from the addition of materialinto the PMMA domains with each cycle. Further, as seen from FIG. 3, theAl₂O₃ cylinder diameter at 10 cycles using 5× longer precursor exposuretimes (timing: 300/300/300/300 seconds) than the above described example(timing: 60/300/60/300) is nearly the same as the cylinder diameterobtained using the shorter precursor exposure times. This furtherdemonstrates that Al₂O₃ in the PMMA domains is self limited. Lengtheningthe exposure time in the cycle from 60 s to 300 s did not have anappreciable impact on the feature size, i.e. the amount of inorganiclayer formed. This also suggests that a reaction exposure of 60 secondsis near the saturation point of the growth; and 300 seconds of purgetime is sufficient to prevent homogenous reactions.

The slope of the line for cylinder diameter in FIG. 3 shows a growthrate of about 2 nm/cycle, which is 16× greater than the Al₂O₃ ALD growthrate of 1.2 Å/cycle on planar surfaces determined by ellipsometry fromthe Si witness samples (lower trace in FIG. 3), such as indicated by theopen squares on FIG. 3. The higher growth rate in the polymer may beascribed to the higher density of accessible reactive sites on thepermeable three dimensional PMMA domains with respect to the solid,planar Si surface. On the other hand, the growth rate of Al₂O₃ cylinderdiameters is smaller than the theoretical value (about 10 nm/cycle)obtained when assuming all Al—OH sites in the polymer matrix areaccessible to the following process. This indicates that sterichindrance from Al—OH and polymer ligands limits the number of accessibleAl—OH sites. With an increasing number of cycles, the growth of Al—Omatrix from different Al—OH seeds will start to coalesce to form a denseand impermeable Al—O network. The growth rate of Al₂O₃ in themicrodomain will then substantially match the growth rate on a solid Siwafer.

In addition to the flexibility in material deposition, there is alsoflexibility regarding the morphology or configuration of the BCPtemplate and, therefore, the ultimate configuration of the inorganicnanostructures. For instance, when the PMMA domains are oriented normalto the scaffold, Al₂O₃ nanoposts may be fabricated. FIG. 2f shows aplurality of nanoposts formed with a diameter of 22.6±2.8 nm, similar tothe in-plane cylinder diameters of 22±2.2 nm observed elsewhere on thesame sample. As such, the present processes could be extended tosynthesize nanomaterials with any of the morphologies accessible toBCPs, even into three-dimensional matrices.

In an embodiment, patterned TiO₂ cylinders were prepared using TiO₂ SISat 135° C. onto self-assembled PS-b-PMMA BCP thin features. In the firstSIS cycle, titanium tetrachloride (TiCl₄, 99.9%) coordinates to thecarbonyl groups as illustrated in FIG. 6b , and then H₂O exposure,hydrolyzes the TiCl₄ to form Ti—OH which serves as the reactive site forsubsequent TiCl₄/H₂O SIS cycles.

Organized patterns of TiO₂ cylinders generated with 5 and 10 cycles ofTiO₂ SIS were visible under SEM as shown by FIGS. 4a and 4b ,respectively. Further AFM measurements confirmed the clear patterns ofTiO₂ cylinders resulting from multiple SIS cycles. X-ray photoelectronspectroscopy (XPS) measurements also confirmed the presence of Ti on thesample surface. As shown in FIGS. 4a and 4b , the domains in lightcontrast are the TiO₂ cylinders with diameters of 13.3±1.4 nm and16.9±1.9 nm after 5 and 10 TiO₂ SIS cycles, respectively. The TiO₂cylinders are smaller than the domain size of the PMMA (about 30 nm) andagain indicate that the growth of the TiO₂ is guided by the moleculartemplate on the polymer chain. The center-to-center distances of theTiO₂ cylinders are nearly identical for these two samples at 55.6±6.6 nmand 59.3±3.2 nm, respectively. These dimensions once again match thecorresponding spacing between the PMMA domains. These results confirmthat the TiO₂ preserves the original pattern that was present in thePS-b-PMMA thin features template.

In another embodiment, nanoscale ZnO patterns was prepared using SIS. AnSIS of ZnO (DEZ, >95% Strem and H₂O at 85° C. and 135° C. with thetiming sequence: 300/300/300/300 seconds) was performed. If moisture ispresent in the PMMA domains, ZnO should form via the hydrolysis reactionbetween DEZ and H₂O. However, inorganic features were not observed bySEM after 5 ZnO SIS cycles followed by O₂ plasma treatment. The AFMheight image shown in FIG. 5c indicates a subtle pattern, suggesting asmall amount of largely non-selective growth induced by the weakinteraction between DEZ and the carbonyl groups in the PMMA domains.This result indicates that a self limited coordination reaction betweenthe metal precursor and the carbonyl groups on the polymer chains isnecessary for seeding the subsequent molecular assembly process by SIS.Owing to the molecular-level engineering offered by this method, in situsynthesized —Al—OH sites in PMMA microdomains formed by treatingPS-b-PMMA with 1 or 2 cycles of Al₂O₃SIS may be utilized to seed thegrowth of other inorganic materials which do not have direct selectivechemistry with pure PS-b-PMMA polymer, e.g., ZnO, MgO, SiO₂, etc.

In yet another embodiment, SIS of ZnO was performed in BCP after firstseeding with 1-2 cycles of Al₂O₃SIS. FIG. 4d shows the AFM image of thepattern resulting from 1 Al₂O₃SIS cycle (timing: 60/300/60/300 seconds)followed by 3 ZnO SIS (DEZ/H₂O) cycles (timing; 300/300/300/300seconds). The pattern is clearer than the image shown in FIG. 4c . Thedifference of height of the cylinders shown in FIGS. 4c and 4d wasconfirmed with corresponding EDAX line scans. The lateral size of thecylinders was measured to be 19.4±3.2 nm and EDAX measurements confirmedthe spatial localization of Zn.

In another embodiment, the process was performed with tungsten (W) SISat 85° C. onto —AlOH seeded PS-b-PMMA scaffold thin features using SISprecursors tungsten hexafluoride WF₆ and disilane Si₂H₆ (WF₆, >99.9% andSi₂H₆ 99.998%, Sigma-Aldrich). FIG. 5a shows the organized nanostructureformed from 1 Al₂O₃ ALD cycle followed by 10 W SIS cycles (timing60/300/60/300 seconds) and an O₂ plasma treatment. Compared with thenanofeatures resulting from 1 SIS Al₂O₃ cycle (FIG. 2a ), the cylindersize is much larger (diameter 25.9±1.9 nm) and broken points are notobserved. Due to the inhibited growth of SIS W on pure polymers, acontrol sample was prepared with 10 W SIS cycles but no Al₂O₃SIS, whichshowed no detectable features by SEM.

Nanocylinders of greater diameter may be generated by performingadditional W SIS cycles, e.g., 20 cycles, as illustrated by FIGS. 5b and5d . As observed in the SEM image in FIG. 5b , the cylinders (diameter41.8±2.7 nm) have a smooth top surface and rougher structures alongtheir sidewalls suggesting that the preferential reaction of W SIS onthe surface of the PMMA domains compared with inside of the domains.This may be due to the relatively slow diffusion of the larger W SISprecursor (WF₆), particularly with increasing numbers of SIS cycleswhich decrease the diameter of the polymer pores and voids. The roughedges of the nanocylinders are ascribed to island-growth behavior of theW with the SIS process. Nevertheless, the gap between the PMMA domainsis clear, which indicates again that the W growth originates within thePMMA domains and progresses as a self-limited heterogeneous surfacereaction.

The presence of W was confirmed by XRF. Moreover, the conductivity ofthe PS-b-PMMA-Al—OH nanocomposite on a SiO₂/Si scaffold after 20 cyclesof W SIS treatment became detectable by two-point I-V measurement,further supporting the incorporation of W metal. As shown in FIG. 5c ,dense nanoposts with dimensions of 40.3±3.1 nm were also observed on thesample shown in FIG. 5b , which again confirms the applicability of theprocess for templating different morphologies of inorganic structures inBCPs. For the sample prepared with 2 cycles of Al₂O₃ and 20 cycles of WSIS, nanocylinder features with larger diameters (43.6±3.1 nm) wereobserved due to the greater density of Al—OH sites. Although thenanocylinders resulting from 20 cycles of W are larger than the PMMAdomain size, the center-to-center spacing of those features, which are55.7±3.2 nm and 55.4±3.0 nm for samples with 1 and 2 cycles Al₂O₃ seeds,respectively, is consistent with the corresponding periodicity in theoriginal PS-b-PMMA features.

The present methods may be extended to include combining a blockcopolymer material incorporating a conjugated semiconducting polymersuch as poly(3-hexylthiophene) (P3HT) with an SIS material that is awide-band-gap semiconductor such as TiO₂ or ZnO. The patternednanostructure would offer a large surface area p-n junction forefficient separation of photo (solar) generated excitons. The compositematerial may be incorporated into organic-inorganic hybrid photovoltaicswhich could achieve enhanced efficiency. Other applications arerecognizable to those of skill in the art and may range from photoniccrystals to various membranes.

The foregoing description of embodiments of the present invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the present invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of thepresent invention. The embodiments were chosen and described in order toexplain the principles of the present invention and its practicalapplication to enable one skilled in the art to utilize the presentinvention in various embodiments, and with various modifications, as aresuited to the particular use contemplated.

What is claimed is:
 1. A nanocomposite organic/inorganic material,comprising: a block copolymer comprising a first polymer and a secondpolymer covalently bonded to the first polymer, the first polymerdefining a plurality of self-assembled ordered microdomains disposedwithin the second polymer, the first polymer including at least onefunctional group absent from the second polymer; and an inorganicmaterial substantially disposed within each of the plurality of orderedmicrodomains, the material comprising at least one metal oxideselectively bound to at least one functional group, wherein the at leastone metal oxide is not bound to the second polymer.
 2. The nanocompositematerial of claim 1, wherein the at least one functional group isselected from a group consisting of: a carbonyl group, an ester group,an ether group, a pyridine group, and a hydroxyl group.
 3. Thenanocomposite material of claim 1, wherein the inorganic materialincludes a second metal oxide selectively bound to a first metal oxide.4. The nanocomposite material of claim 1, wherein the first polymercomprises a hole-transport polymer and the inorganic material ischaracterized by a semiconductor, and wherein the nanocomposite materialis integrated in an organic-inorganic hybrid photovoltaic device.
 5. Thenanocomposite material of claim 1, wherein the first polymer comprisesan electron- or ion-conducting polymer and wherein the inorganicmaterial provides structural support for a separation membrane in abattery or capacitor.
 6. A nanocomposite organic/inorganic material,comprising: a block copolymer including a first polymer free of areactive functional group and a second polymer forming a plurality ofordered polymer nanoscale domains characterized by the reactivefunctional group and having a configuration; and an inorganic materialsubstantially disposed within the configuration of the second polymer,the inorganic material comprising at least one metal oxide selectivelybound to the reactive functional group and forming a plurality ofinorganic features, wherein the at least one metal oxide is not bound tothe first polymer.
 7. The nanocomposite material of claim 6, wherein theat least one reactive functional group is selected from a groupconsisting of: a carbonyl group, an ester group, an ether group, apyridine group and a hydroxyl group.
 8. The nanocomposite material ofclaim 6, wherein the inorganic material includes a second metal oxideselectively bound to a first metal oxide.
 9. The nanocomposite materialof claim 6, wherein the first polymer comprises a hole-transport polymerand the inorganic material is characterized by a semiconductor, andwherein the nanocomposite material is integrated in an organic-inorganichybrid photovoltaic device.
 10. The nanocomposite material of claim 6,wherein the first polymer comprises an electron- or ion-conductingpolymer and wherein the inorganic material provides structural supportfor a separation membrane in a battery or capacitor.
 11. Thenanocomposite material of claim 6, wherein the plurality of orderedpolymer nanoscale domains' configuration is a plurality of cylinders orinverse cylinders and the plurality of inorganic features are aplurality of metal cylinders of the first metal.
 12. The nanocompositematerial of claim 11, wherein the plurality of metal cylinders are anarray emitter or a field emitter.
 13. The method of claim 11, whereinthe plurality of metal cylinders form a three-dimensional electricalcontact infiltrating organic semiconductor features.